Abstract
Boundary layer transition is a relevant phenomenon in many aerodynamic and aerothermodynamic problems and has been extensively investigated from the past century till recent times. Among the factors affecting the transition process, surface roughness plays a key role. When a roughness element with sufficiently large height (h) compared to the boundary layer thickness (δ) is immersed in a laminar boundary layer, it will produce spanwise varying disturbances with the potential to accelerate the transition process. In the thesis, a fundamental study is carried out to understand the physical mechanism of isolated roughness element induced transition. Experiments are performed in incompressible flow regime covering both critical and supercritical conditions. Tomographic particle image velocimetry (PIV) is employed as the main experimental diagnostic technique, returning the threedimensional velocity and vorticity field of the flow.
The threedimensional wake flow behaviour is firstly identified behind roughness element of microramp geometry. The microramp produces a pair of counterrotating streamwise vortices in the wake, transporting low momentum fluid away from the wall by the central upwash motion, and sweeping the high momentum flow towards the nearwall region sideward. The shear layer around the central lowspeed region is related to the growth of KelvinHelmholtz (KH) instability. The active range of the primary vortices and the central lowspeed region in the streamwise direction is associated to the selection of the dominant instability mechanism, which decreases with the increase of roughnessheight based Reynolds number (Re_{h}).
The instantaneous flow field reveals that the earliest unstable structures featuring hairpin shape are caused by the KH instability at the separated shear layer. The evolution of KH vortices is strongly influenced by Re_{h}. At Re_{h} = 1170, the KH vortices are lift up under the upwash motion effect of the quasistreamwise vortices, following by paring, distortion and finally breakdown. The active region of KH vortices is separated from the inception of turbulent wedge, where early stage transition occurs. When Re_{h} decreases approaching the critical value, the KH vortices progressed gradually until the overall shear layer is destabilized, indicating the correlation between KH instability and transition. The POD analysis yields the symmetric (KH) and asymmetric mode. The disturbance energy associated to the symmetric modes changes with Re_{h}. At higher Re_{h}, the disturbance energy of the symmetric modes quickly decays, having a comparable contribution as the asymmetric modes. When Re_{h} < 1000, the symmetric modes produce a remarkably higher level of disturbance energy until the onset of transition, indicating its dominance.
The effectiveness of roughness element on promoting transition is strongly influenced by its geometry. The blufffront roughness elements induce horseshoe vortices due to upstream separation. The different rotation direction of these vortices compared to the microramp leads to early inception of sideward growth of fluctuations, and more rapid transition process. While for the slender microramp, significant longer distance is required to for the onset of transition.
The threedimensional wake flow behaviour is firstly identified behind roughness element of microramp geometry. The microramp produces a pair of counterrotating streamwise vortices in the wake, transporting low momentum fluid away from the wall by the central upwash motion, and sweeping the high momentum flow towards the nearwall region sideward. The shear layer around the central lowspeed region is related to the growth of KelvinHelmholtz (KH) instability. The active range of the primary vortices and the central lowspeed region in the streamwise direction is associated to the selection of the dominant instability mechanism, which decreases with the increase of roughnessheight based Reynolds number (Re_{h}).
The instantaneous flow field reveals that the earliest unstable structures featuring hairpin shape are caused by the KH instability at the separated shear layer. The evolution of KH vortices is strongly influenced by Re_{h}. At Re_{h} = 1170, the KH vortices are lift up under the upwash motion effect of the quasistreamwise vortices, following by paring, distortion and finally breakdown. The active region of KH vortices is separated from the inception of turbulent wedge, where early stage transition occurs. When Re_{h} decreases approaching the critical value, the KH vortices progressed gradually until the overall shear layer is destabilized, indicating the correlation between KH instability and transition. The POD analysis yields the symmetric (KH) and asymmetric mode. The disturbance energy associated to the symmetric modes changes with Re_{h}. At higher Re_{h}, the disturbance energy of the symmetric modes quickly decays, having a comparable contribution as the asymmetric modes. When Re_{h} < 1000, the symmetric modes produce a remarkably higher level of disturbance energy until the onset of transition, indicating its dominance.
The effectiveness of roughness element on promoting transition is strongly influenced by its geometry. The blufffront roughness elements induce horseshoe vortices due to upstream separation. The different rotation direction of these vortices compared to the microramp leads to early inception of sideward growth of fluctuations, and more rapid transition process. While for the slender microramp, significant longer distance is required to for the onset of transition.
Original language  English 

Qualification  Doctor of Philosophy 
Awarding Institution 

Supervisors/Advisors 

Award date  15 Jun 2017 
Print ISBNs  9789461868220 
DOIs  
Publication status  Published  2017 
Keywords
 Isolated roughness
 Boundary layer
 Transition
 Stability
 Vortical structures